Springer Old Growth Forests - Chapter 11 potx

Chapter 11 Soil Carbon Accumulation in Old-Growth Forests Gerd Gleixner, Cindy Tefs, Albrecht Jordan, Matthias Hammer, Christian Wirth, Angela Nueske, Alexander Telz, Uwe E. Schmidt, and Stephan Glatzel 11.1 Introduction An area of 4.1 billion ha land is covered with boreal, temperate and tropical forest, together comprising up to 80% of the terrestrial aboveground carbon and 40% of total soil carbon (Dixon et al. 1994; Pregitzer and Euskirchen 2004). Forest ecosystems are well studied, mainly because of their importance for timber produc- tion during the early economic development of many countries. In the context of global change, however, other ecosystem serv ices like provision of drinking water or carbon sequestration have gained importance. Less is known about these. For ecosystem carbon uptake, it is assumed that biomass production is highest in younger and middle-aged stands but declines with forest age (Pregitzer and Euskirchen 2004) and that long-term soil carbon sequestration is very low (Schlesinger 1990). Both factors suggest that old-growth forests are close to being carbon neutral, i.e. neither storing nor losing carbon. However, these assump- tions neglect the fact that root and leaf litter production and the accumulation of coarse woody debris might be highest in old-growth forests, and that soil carbon storage might occur in deeper soil layers rather than in the more often investigated top soils. This chapter will summarise current knowledge regarding soil carbon storage, identifying factors that might affect soil carbon storage in old-growth forests. Finally, the first results relating to soil carbon storage from a case study in a 250-year-old beech forest will be presented. 11.2 Development of Soil Carbon Stocks in Ecosystems In the long term, accumulation of soil carbon during ecosystem development is driven by the input, decomposition and output of plant-derived carbon. The initial step for most carbon found in soil is autotrophic reduction of oxidised carbon from the atmosphere by plants using energy provided by the sun. In the early stages of ecosystem development during primary succession, e.g. after the retreat of ice in the C. Wirth et al. (eds.), Old‐Growth Forests, Ecological Studies 207, 231 DOI: 10.1007/978‐3‐540‐92706‐8 11, # Springer‐Verlag Berlin Heidelberg 2009 late Pleistocene, mainly lower plants such as lichens and mosses produce these reduced carbonaceous compounds and add them to the bare surface. As a conse- quence, surface rocks are biologically weathered and nutrients for plant growth are provided (Barker and Banfiel d 1996). First, soil organic matter (SO M) is formed from decomposing biomass and increases the water holding capacity of the surface. Increased nutrient availability and water holding capacity in parallel with tempera- ture increases accompanying ongoing deglaciation, improve growth conditions for plants and enable further progress in ecosystem development, which can be seen in the development of different plant and animal communities and in the formation of soil profiles. Increasing biomass, and therefore litter production, form a litter layer (Oi horizon) of poorly decomposed ‘‘fibric’’ plant litter (Fig. 11.1). Underneath this litter layer, an organic layer of partially degraded, fermented or ‘‘hemic’’ (Oe horizon) plant material develops. No plant structures can be identified in the humic or ‘‘sapric’’ horizon (Oa horizon) above the surface of the mineral layer. Organic matter is also transported into deeper mineral soil layers either by biotur- bation or by percolating rainwater. The latter process is critically important for the development of soil profiles and might also enhance carbon storage in the long term. The transport of organic carbon from the O horizons into the upper mineral soil and, in parallel, the export of minerals and metal oxides from the upper mineral soil through percolating soil water form a mineral-depleted A horizon in the upper mineral soil. Below the A horizon, an often brownish or reddish mineral-enriched B horizon forms due to the precipitation of leached weathering products, i.e. iron oxides/hydroxides and/or humic substances, from the percolating stream of soil water. Underneath the developed soil profile the unaltered parent substrate remains in the C horizon. The main initial sources for soil organic matter in natural systems are leaf litter input to the soil surface and root litter and exudate inputs within the soil profile. As a global average, over 60% of the root biomass is found in the top 20 cm of the soil (Fig. 11.2). Roo t biomass decreases logarithmically with depth and only 14% is found below 40 cm. The gradient in soil carbon is much smaller. Only 40% organic carbon is located in the top 20 cm soil, and this also decreases logarithmically; however, 36% soil carbon is still found at depths below 40 cm and therefore soil carbon is enriched relative to root biomass (as a proxy of root input) with soil depth. The non-linear distribution pattern of root biomass and soil carbon may result on the one hand from the depth-depending decomposition of biomass and increased substrate use-efficiency of soil microorganisms in deeper soil layers (Witter and Kanal 1998), and on the other hand from the higher sorption capacity of ‘‘cleaner’’ mineral surfaces at deeper soil layers. The spatial variability of root input in the soil of a forest stand is known to be influenced inter alia by water availability, nutrient status, stand density and species composition. Stand age per se has no direct effect on the vertical and horizontal distribution of roots (see Chap. 10 by Bauhus, this volume) and might therefore only indirectly influence soil carbon accumulation. In the upper 20 cm soil profiles, the decomposition of biomass, and hence the decomposer community, i.e. the soil macro-, meso- and micro-organisms, appears to exert a stronger control on carbon storage. In general, soil organisms decompose 232 G. Gleixner et al. plant input like litter and root exudates, and release most of the assimilated carbon back to the atmosphere again as CO 2 . Some of the plant-derived plant litter remains untouched above the soil, i.e. acid-generating conifer litter forming raw humus, but most of the litter-derived carbon remaining in the soil is transformed to SOM by the action of soil organisms (Gleixner et al. 2001). The complex process of SOM formation is achieved by the trophic networks (Ekschmitt et al. 2008) in the soil and can be influenced by the composition of the decomposer community, which in B Fig. 11.1 Terminology of soil horizons in a depth profile. Oi, Oe and Oa organic layers; A mineral layer with organic carbon and leached minerals; B mineral layer with precipitation of oxides/ hydroxides and/or carbon; C unaltered parent substrate. Arrows indicate the decreasing water flow down the soil profile 11 Soil Carbon Accumulation 233 turn might be influenced by stand age. In general, shredding organisms, like earth- worms or woodlice, break litter into small pieces and extract digestible compounds. This process increases the surface area of litter and inoculates decomposing micro- organisms, which degrade indigestible compounds externally (Gleixner et al. 2001). Soil animals like nematodes, woodlice, collembola or mites feed on these nutrient-rich microorganisms and predators hunt microbe-feeding soil animals in the soil. Finally, decomposers mineralise dead soil animals, closing the element cycle of carbon in soil. In summary, the formation of soil carbon depends on (1) the amount, quality and distribution of input material; (2) the activity of decomposers and the decom- position rate; and (3) carbon transport to deeper soil layers. 11.3 Soil Carbon Storage in Old-Growth Forests 11.3.1 Effects of Quantity and Quality of Input Material In general, the stock of carbon in soils is correlated to the mean annual temperature and the mean annual precipitation, and thus indirectly to net primary production (NPP) (Amundson 2001). Sun et al. (2004) analysed 36 forest stands from three Fig. 11.2 Distribution of soil carbon and root biomass in depth profiles of the world’s major ecosystems. y Error bars sampling interval, x Error bars standard deviation from 11 biomes summarising 2,721 soil samples and 117 root biomass samples (Jobbagy and Jackson 2000) 234 G. Gleixner et al. forest sites in Oregon with NPP ranging from 180 to 1,200 g C m –2 year –1 . They found a tight relationship between NPP and carbon stored in the soils across sites but not within sites (Fig. 11.3). Thus, this trend was driven mostly by the difference in NPP between sites, which in turn was correlated to the amount of precipitation supplied to the different ecosystems. No effect of stand age on soil carbon stocks could be detected within sites. Along the chronosequences, high initial soil carbon stocks were lost in young stands but increased again in two out of the three cases in old growth forests (Sun et al. 2004). Only the site with the lowest productivity also lost carbon in the mature stand. The authors suggested that legacy carbon is decomposed and de novo carbon is formed as a consequence of ecosystem development. It was concluded that the ratio of necromass carbon to total ecosystem carbon decreases with stand age and remains constant in old-growth forests. However, the oldest stands per site consistently exhibited increasing ratios per site, suggesting a continuous necromass build-up (Sun et al. 2004). Such a build-up of necromass could be driven by the litter quality, e.g. lignin is thought to be more stable to microbial decomposition than cellulose. Comparing three different sites with Douglas fir in Oregon, each comprising an chronose- quence of young stands, secondary forest and old-growth forest, Entry and Emmingham (1998) found consistent age-related trends in the composition of litter and SOM. Litter in young stands contained up to 80% structural carbohydrates, i.e. cellulose (Fig. 11.4). This contribution decreased with age and old-growth forest litter contained only about 40% structural carbohydrates. At the same time, the contribution of lignin increased from less then 10% in young stands to 40% in old- growth stands. This change in the chemical composition of the litter layer coincides with the higher content of twigs and reprodu ctive structures. The input of litter almost doubled, from 200 to 400gm –2 , from a 20-year-old stand to an old-growth stand (Klopatek 2002). It follows that higher amounts of less degradable input may be provided in old-growth forests and this could benefit soil carbon storage. Soil organic matter, however, did not follow the chemical trend observed for litter Fig. 11.3 Effect of net primary production (NPP) on the formation of soil carbon (Sun et al. 2004). Sampling sites: CH Cascade Head, OR; HJ HJ Andrews LTER site, OR; ME Metolius; OR 11 Soil Carbon Accumulation 235 (Entry and Emmingham 1998). As in the litter, structural carbohydrates, such as cellulose and hemicellulose, also decreased slightly with age in old-growth forests from 60% to 40%; however, lignin was not affected and remained constant at about 20% in all three age classes (Fig. 11.4). Most striking was the increase in non- structural carbohydrates with age from 20% to 40% in old-growth forests. The origins of these non-structural carbohydrates is unclear, but they are most likely constituents of bacterial cell walls (Gleixner et al. 2001). The chemical similarity of SOM of different age is supported by results from mass-spectrometric investiga- tions (Hoover et al. 2002). Comparing SOM from a chronosequence after stand- replacing fires with an > 600-year-old virgin beech-hemlock site in Pennsylvania, no difference in the chemical composition between the virgin and the youngest site could be detected. In the upper 30 cm soil, a clear trend of decreasing recent, i.e. plant-related, carbon and an increase in humified carbon was observed. Results from litter studies suggest that, in the long term, the amount of input carbon drives soil carbon accumulation. The chemical composition of input carbon is of minor importance as all plant-derived chemical structures can be decomposed and trans- formed by soil microorga nisms into SOM. However, environmental conditions such as acid-ge nerating conifer litter or water-saturated soil can influence the decomposer community and decomposition processes, and hence litter accumula- tion followed by lower carbon input into the mineral soil and the build-up of organic layers can be expected. Relative content (%) Fig. 11.4 Composition of soil organic matter relative to forest age class (Entry and Emmingham 1998) 236 G. Gleixner et al. 11.3.2 Effects of Organic Matter Decomposition and Soil Respiration A wide range of methods is available to measure the decomposition of organic matter. Mechanistic approaches measuring the activity of different enzymes (Sinsabaugh et al. 1991; Stemmer et al. 1999; Ekschmitt et al. 2008), balance methods using litter bags (McClaugherty et al. 1985; Joergensen 1991; Smith et al. 2002), differences in the total litter layer (Vitousek et al. 1994), or respiration methods (Zimov et al. 1996; Gower et al. 1997; Janssens et al. 2000; Pumpanen et al. 2004) can be applied. Each type of method has specific advantages and disadvantages. The most integrative values for decomposition of SOM and litter are obtained using soil respiration measurements. The major disadvantage of this method is the additional contribution of root and rhizospheric respiration. Given the fundamental importance of respira- tion processes for total ecosystem carbon balance and for the global carbon balance of the atmosphere (Houghton and Woodwell 1989; Raich and Schlesinger 1992; Schimel 1995), we will review the current literature on soil respiration in order to evaluate the effect of decomposition on soil carbon storage. 11.3.2.1 Soil Respiration in Forest Ecosystems and Within-Site Spatial Heterogeneity Like productivity or total soil carbon content, soil respiration is related to climatic gradients. Cold or dry biomes like tundras or deserts have the lowest mean rates of soil respiration at between 60 and 220 g C m –2 year –1 . Ecosystems with high temperatures and high moisture availability like tropic al rain forests have the highest rates, i.e. in the order of 2,000 Æ 1,000 g C m –2 year –1 (Raich and Schle- singer 1992; Adachi et al. 2006; Sotta et al. 2006). Consequently, we compared the major controls on biome-specific soil respiration rates for (sub-) tropical, temperate and boreal forest. Varying soil respiration rates within the same biome, and even within the same measurement site, are commonly observed (Raich and Schlesinger 1992). This spatial heterogenei ty in soil respiration causes high uncertainty of total annual fluxes. Several factors are known to contribute to this heterogeneity, e.g. high variability of soil structure (Bouma and Bryla 2000), soil moisture (Rapalee et al. 1998), bacterial and fungal distribut ions (Go ¨ mo ¨ ryova ´ 1994), root density (Hanson et al. 2000; M. Mund et al., manuscript in preparation), SOM content, wind speed at the soil surface and pressure patterns (Janssens et al. 2000; Martin and Bolstad 2005). The importance of each factor may be site-specific, biome-specific and even age-dependent. Unfortunately, knowledge of age trends relative to soil respiration is very sparse. Only Campbell and Law (2005) have investigated soil respiration across three climatically distinct chronosequences at four different age classes, but age-related trends were not consistent between forest types. However, in order to estimate the decomposition rate for different sites and differentially aged 11 Soil Carbon Accumulation 237 stands, it is important to gain appropriate knowledge of the ‘‘within-site’’ heteroge- neity of soil respiration. In the fol lowing paragraphs, we assess the importance of factors that control soil respiration and summarise the implications of spatial heterogeneity of soil respiration for old-growth forest carbon balances. In tropical and subtropical forest soils, water content is suggested to be the main driver of the variability in soil respiration (Sotta et al. 2006). This can be caused either directly by both topographical featur es and the size distribution of soil particles influencing the water content, or indirectly by the water-dependent distri- bution of roots and decomposing microorganisms. The main mechanisms seem to involve the fact that these high rainfall biomes sites have higher water contents as a consequence of high precipitation, thus leading to lower oxygen influx. The lack of oxygen prevents root growth and suppresses microbial decomposit ion, and there- fore is associated with slower CO 2 efflux rates. Therefore, decomposition is higher at drier sites and carbon accumulation might occur at wetter sites. In temperate coniferous, broad-leaved and mixed forest, soil respiration seems to be driven primarily by the amount of fine roots of trees and understorey (M. Mund et al., manuscript in preparation). Although soil respiration near the trees is higher in young stands than in old stands, due to the higher root biomass the total soil respiration is higher in old stands (Soe and Buchmann 2005). Furthermore, soil respiration is positively influenced by the amount of carbon available for decomposition, whereas high soil moisture reduces the soil respiration rate. In contrast to the tropical syst em, low water content in summer often slows down root respiration and microbial activity (Saiz et al. 2006). In boreal forest, soil respiration is driven mostly by the amount and C/N ratio of the litter or the underlying brown moss layer, highlighting the importance of litter layers for boreal ecosystems (Rayment and Jarvis 2000). The loss of the litter layer due to disturbances like fire generally leads to lower respiration rates (Shibistova et al. 2002). Like in tropical forests, sites with high soil moisture content, or even with anaerobic site conditions, have lower respiration rates (Rayment and Jarvis 2000). Higher temperatures in summer increase spatial variability in soil respira- tion; however, this effect was due mostly to higher root activity and not temperature effects per se (Khomik et al. 2006). 11.3.2.2 Heterotrophic Respiration in Old-Growth Forests In order to overcome the uncertainty of soil respiration introduced by the high spatial variability that is mostly induced by autotrophic contributions related to roots and the low temporal coverage of respiration measurements, heterotrophic respiration may be calculated from the difference between independently measured NPP and net ecosystem productivity (Pregitzer and Euskirchen 2004). For boreal, temperate and tropical ecosystems, the estimated amount of annual heterotrophic respiration was slightly lower than soil respiration measurements in the corresponding ecosystems (see above). This discrepancy might be due to different scaling methods. Most interestingly, Pregitzer and Euskirchen (2004) observed a 238 G. Gleixner et al. continuous decline in heterotrophic respiration with increasing stand age classes (Fig. 11.5). They suggested that disturbances associated with stand replacement, like fire or harvest, caused high heterotrophic respiration rates in young stands and that this legacy effect levels off in old-growth stands. This is supported by similar respiration rates of girdled and non-girdled trees 2 years after girdling (Ekberg et al. 2007). Unfortunately, no direct observations along chronosequences are available to support this observation. However, the decline in respiratory losses from young stands to old-growth forests would overcompensate for the decline in NPP, and suggests additional carbon is available for sequestration or drainage. 11.3.3 Drainage of Dissolved Carbon from Forest Ecosystems Losses of dissolved or particulate carbon with precipitation percolating to the groundwater might be an important process, either to transport carbon to deeper soil layers for storage or for the removal of carbon from the ecosystem. The latter process was reviewed for 42 forest ecosystems having temperate, boreal or alpine climates and covering all major soil types (Michalzik et al. 2001). Both conifer and broadleaf forests were analysed; however, no age-dependent data were used. The highest carbon losses, of between 10 and 40 g C m –2 year –1 , were found underneath the Oa horizon, supporting the notion that decomposition of leaf and root litter is the main source of dissolved carbon losses (Ekberg et al. 2007; Uselman et al. 2007). However, the total amount of litter or coarse woody detritus [see Chaps. 5 (Wirth and Lichstein) and 8 (Harmon), this volume], which is higher in old forest, was negatively correlated to carbon export, suggesting enhanced gaseous carbon losses due to priming of microbial decomposition (Steinbeiss et al. 2008a). The total export rate of dissolved carbon decreased strongly in the mineral A and B horizons , and less then 10% carbon transferred from the Oa horizon was exported to the unaltered parent material, i.e. to the C horizon (Michalzik et al. 2001). In a two- phase sorption equilibrium, carbon is reactively transported to deeper soil layers. Fig. 11.5 Heterotrophic respiration across all age classes in boreal, temperate and tropical biomes (Pregitzer and Euskirchen 2004) 11 Soil Carbon Accumulation 239 Carbon is thereby partly transformed to SOM and partly respired by soil micro- organisms (Steinbeiss et al. 2008a). In total, dissolved losses of carbon from forested upland ecosystems are rather small and almost negligible. Only one study could be found that investigated dissolved c arbon exports in relation to stand age. Peichl et al. (2007) studied a chronosequence of white pine (Pinus strobus ) afforestations in southern Ontario starting from carbon-depleted agricultural land. The annual export of dissolved organic carbon decreased from an initial 7 g C m –2 year –1 2 years after afforestation to 2 g C m –2 year –1 in 65-year old sites. These data suggest that losses of dissolved carbon in old-growth forests are negligible. 11.3.4 Soil Carbon Stock Changes As with soil respiration, the large spatial variability in soil carbon strongly limits the detection of carbon stock changes in the soil. In addition to changes in the carbon concentration of mineral soil, changes in soil bulk density also have to be considered. The latter, however, iscontrolled mostly byphysical processes like swelling, shrinking or freezing, by biological processes like digging soil fauna or penetrating roots, or by chemical factors like the total concentration of carbon in the soil. Most of these factors change over the course of the year and are difficult to compare. Therefore changes in carbon concentration have proven to better reflect changes incarbon stocks (Steinbeiss et al. 2008b). Additionally, time series investigating changes at identical sites are very rare (Sect. 11.4; Zhou et al. 2006; Kelly and Mays 2005). Pregitzer and Euskirchen (2004) compared carbon stocks determined for differ- ent forest stands separated according to age classes. In general, for temperate, boreal and tropical soil, they consistently found mean carbon stocks in the order of 10,000 g C m –2 . In boreal and tropical forests, the carbon stocks of young stands were 10% and 50% lower, respectively, than the mean across all age classes. In temperate forests, 10-year-old stands had slightly higher carbon stocks that initially decreased and then started increasing again at a stand age of about 30 years. The highest carbo n stocks were always found in the oldest stand age class. This eff ect was strongest in boreal systems where, on average, the soil carbon stocks found in old-growth forests were twice those found in young stands. The analysis of Preg- itzer and Euskirchen was the first systematic global meta-analysis of age-related changes in carbon stocks but has two main limitations: First, stands from individual investigations and chronosequences were pooled into broad age classes irrespective of site quality and hydrology. The results are therefore influenced by the interaction between site quality and age. For example, forests on poor soils develop more slowly and therefore tend to dominate the older age classes. This potentially introduces a bias towards lower accumulation rates in old forests. Second, the age range was rather limited and, in fact, for temperate and tropical forest no data from stands older than 200 years were included. Third, differences in the depth to which the carbon stocks were quantified were not corrected for. In the following, we present a meta-analysis that avoids these problems. Using data from the literature, we take two approaches. In a first step (age-class approach), 240 G. Gleixner et al. [...]... the tropical sequences in Fig 11. 7 compare only young and old- growth systems, often without indicating the exact age of the old- growth stage (in which case it was assigned an age of 300 years according to Wirth et al., Chap 2, this volume) A linear interpolation probably misses non-linear dynamics along the sequences, and it is likely that this approach underestimates old- growth carbon sequestration... such as fire or erosion Nevertheless, as a whole, the data suggest that the soil in forests remains a small-to-moderate carbon sink well into the old- growth stage The few estimates based on repeated sampling (Sect 11. 4; Homann et al 2005; Kelly and Mays 2005; Zhou et al 2006) even suggest substantial rates in oldgrowth forests that may be in the range of the aboveground litterfall It should be noted... ‘transition’, ‘early old growth and ‘late old growth (see text) The intersections between smooth lines and vertical lines were used to calculate the changes in biomass carbon stocks during the successional stages The numbers 1 24 indicate the sequences described in detail in Table 11. 2 Points in brackets indicate stands that were labelled old growth ; these were assigned the mean age of old growth sites... stock changes in old- growth forests are geo-referenced soil samplings repeated at the same location and with a depth resolution of at least 1 m in order to investigate the effect of soil profile development and potentially important carbon storage in deeper soil layers An example of such a study is given in the next section 11. 4 Case Study of Soil Carbon Sequestration in a 250-Year -Old Beech Forest... often below the investigated soil depth These results from the improved age-class approach suggest that temperate deciduous and tropical evergreen forests continuously accumulate soil carbon until the highest age-class (>190 or 200 years) Conifer-dominated boreal and temperate forest potentially also accumulate carbon in old- growth forests, but here carbon is found in thick organic soil layers that are... al (2002)g Median Æ SD Transition stage Chronosequences (Sect 11. 3.4) Pregitzer and Euskirchen (2004)f Homann et al (2005)h Trumbore and Harden (1997)i Median Æ SD Early old growth stage Chronosequences (Sect 11. 3.4) Kelly and Mays (2005)j Case study (Sect 11. 4) Late old growth stage Schlesinger 1990 Zhou et al 2006k Chronosequences (Sect 11. 3.4) Median Æ SD Biomea Methodb Soil depth (cm) c Carbon storage... et al 2002 Reference Table 11. 2 Boreal, temperate and tropical forest chronosequences of soil carbon extending beyond a stand age of 150 years Labels refer to the panel numbers in Fig 11. 7 DC Change in soil carbon stocks (g C m–2 year–1), Pioneer pioneer phase (1–100 years), Transition transition phase (101–200 years), EOG early old- growth phase (201–400 years), LOG late old- growth phase (401–600 years),... repeated sampling is not enough and more such studies are needed to quantify carbon sequestration rates in old- growth forests under undisturbed conditions and to resolve the above-mentioned discrepancies Several mechanisms have been suggested that allow soil carbon sequestration during the old- growth stage These range from a deterioration in litter quality, increases in litter production rates, to progressive... 0.031), respectively Forests dominated by boreal conifers lost up to 24% mineral soil carbon with age; however, this age-trend of carbon stocks was not significant No clear pattern emerged for temperate coniferous forests Initially, these forests gained up to 20% carbon, but it was lost again in the oldest age class (Fig 11. 6) The decline in mineral soil carbon stocks in boreal forests is probably due... (tropical) Data for the late old- growth phase are too scarce to allow any interpretation 252 G Gleixner et al It should be noted here that these chronosequence-based estimates of DCSOM are much lower than those estimated with other methods (cf Table 11. 4) Comparing developmental phases within biomes, there was no clear age-trend for the chronosequences of boreal and temperate broad-leaved biomes In the temperate . 200 to 400gm –2 , from a 20-year -old stand to an old- growth stand (Klopatek 2002). It follows that higher amounts of less degradable input may be provided in old- growth forests and this could benefit. constant in old- growth forests. However, the oldest stands per site consistently exhibited increasing ratios per site, suggesting a continuous necromass build-up (Sun et al. 2004). Such a build-up of. the activity of decomposers and the decom- position rate; and (3) carbon transport to deeper soil layers. 11. 3 Soil Carbon Storage in Old- Growth Forests 11. 3.1 Effects of Quantity and Quality of